1. Field of the Invention
The invention relates to a circuit arrangement for deriving a control signal from a synchronising signal whose phase alternates in a predetermined sequence. The control signal is independent of the predetermined phase alternation. The arrangement comprises a reference oscillator for supplying a reference signal of a specific frequency and phase, a phase-comparator means for generating a phase signal corresponding to the phase difference between the synchronising signal and the reference signal, a correction circuit for generating a correction signal of predetermined amplitude and a polarity dictated by the phase signal, and a logic combination device for superimposing the phase signal and the correction signal on the control signal so as to cancel control-signal variations caused by the phase alternation of the synchronising signal.
2. Prior Art
DE-PS 32 02 210 describes a device for processing a PAL color video signal, in which a reference output signal supplied by a reference oscillator is applied to a phase comparator which also receives a chrominance signal of carrier frequency. The phase comparator compares the phase of the carrier-frequency chrominance signal with the phase of the reference output signal of color-subcarrier frequency. The phase comparator effects phase comparison between the carrier-frequency chrominance signal and the reference output signal during the color-burst period. The output signal of the phase comparator is applied to a variable-frequency oscillator via an adder and low-pass filter as a control signal to control the frequency of this oscillator.
When a PAL color video signal is processed, the phase of the color-burst signal applied to the phase comparator in a horizontal scanning period has changed about 90.degree. in comparison with that in the next horizontal scanning period and the circuit arrangement operates in such a way that the average phase of the color-burst signal is in synchronism with the phase of the reference output signal. This results in phase errors appearing on the output of the phase comparator, which errors correspond to a phase shift of approximately 45.degree. and have different polarities in alternate horizontal scanning periods. If the signal from the output of the phase comparator is applied directly to the variable-frequency oscillator via the low-pass filter its frequency will change every line, thereby introducing a phase variation of the carrier-frequency chrominance signal in said line.
Therefore, the known circuit arrangement is provided with a correction circuit in order to preclude such an undesired chrominance variation in the lines. This correction circuit comprises a circuit for extracting, processing and dividing the horizontal synchronizing signal contained in the color video signal to produce a continuous pulse signal of half the horizontal frequency. This pulse signal is applied to a current source via a phase inverter to control the polarity of a constant direct current supplied by this current source. In the adder this direct current is superimposed on the output signal of the phase comparator.
In order to ensure that a current of the correct polarity is superimposed on the output signal of the phase conparator, i.e. in such a way that components of half the line frequency contained in said output signal are cancelled, the control signal is applied from the output of the low-pass filter to a pulse counter via a high-pass filter, a half-wave rectifier and a pulse shaper, which pulse counter counts the pulses appearing on the output of the low-pass filter. When the counter has counted four pulses this causes a transition from the "0" level to the "1" level in the output signal of the pulse counter. This transition triggers a flip-flop, which changes over a phase inverter by means of which the polarity of the signal of half the horizontal frequency and hence the polarity of the current supplied to the adder by the current source is changed. By means of a signal of 1/16 horizontal frequency derived from the signal of half the horizontal frequency, the pulse counter is reset to its initial state at regular intervals.
The known circuit arrangement is very intricate and does not operate reliably. Moreover, in the event of a fault condition, i.e. in the case that the polarity of the current supplied by the current source is not in conformity with the output signal of the phase comparator, at least eight horizontal periods are needed to correct the polarity of the current and thus to eliminate chrominance errors in the known circuit arrangement. Such a fault in a picture reproduced by means of the color video signal is very annoying to the viewer.